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STORY: Thousands of demonstrators supporting the army gathered on Friday in Cairo's Tahrir Square, the focal point of two years of turmoil in Egypt.
Pro-army demonstrations were planned across Egypt, while backers of deposed president Mohamed Mursi and his Muslim Brotherhood also announced widespread rallies, with 34 in the Cairo area alone.
Army chief General Abdel Fattah al-Sisi has urged Egyptians to demonstrate against "violence and terrorism" on Friday, while supporters of Mursi, toppled by the military on July 3, called for counter-rallies.
On Friday, Egypt's state news agency said Mursi is under investigation for an array of charges including murder.
Mursi has not been seen in public since his downfall and the army has said he is being held for his own safety. But Mena news agency said the former president would now be detained for 15 days as a judge investigated a raft of allegations.
The probe centers on charges that he conspired with the Palestinian Islamist group Hamas to flee jail during the 2011 uprising against veteran autocrat Hosni Mubarak, killing some prisoners and officers, kidnapping soldiers and torching buildings.
Mursi has previously said locals helped him escape from prison during the 2011 upheavals and the Muslim Brotherhood denounced the series of accusations leveled against him.
Supporters of the deposed Islamist leader staged counter demonstrations to demand his reinstatement, shrugging off fears of an imminent crackdown and vowing not to give in to an army demand for an immediate end to their protests.
Many thousands of men, women and children joined Brotherhood supporters at their round-the-clock vigil in northeast Cairo.
Confrontation appeared inevitable following a month of clashes in which close to 200 people, mainly supporters of Mursi, have been killed.